1. Field of the Invention
The present invention relates to a fabrication method of a semiconductor device and more particularly, to a fabrication method of a semiconductor device containing a formation process of a refractory metal silicide film on a semiconductor material, which is preferably applicable for fabrication of a metal-oxide-semiconductor field-effect transistor (MOSFET) on a silicon substrate.
2. Description of the Prior Art
In recent years, semiconductor devices such as very-large-scale integrated circuit devices (VLSIs) have been finely-structured and large-scale integrated more and more so that the submicron-order fine line technology has been popularly adopted. For this reason, a problem that the circuit operation of the VLSI is delayed due to sheet resistance increase has occurred.
For example, with an MOSFET, both a gate electrode and a pair of diffusion regions forming source/drain regions have been increasingly reduced in size. As a result, the circuit operation using the MOSFET has tended to be delayed due to the sheet resistance increase of conductor films employed in the MOSFET.
To solve such the problem in the finely-structured MOSFET, various resistance-reduction techniques using refractory metal silicides were developed and employed. More recently, as one of the resistance-reduction techniques, the application of a self-aligned silicide film (SALICIDE technique) containing titanium (Ti) to the MOSFET has received much attention.
In addition, with such finely-structured MOSFET, the source/drain diffusion regions needs to be formed shallower in a semiconductor substrate and at the same time, the Ti silicide film applied thereto needs to be thinner in order to restrain an electric current leakage.
In the above SALICIDE technique, the Ti silicide (TiSi.sub.2) film is, in general, produced by utilizing two solid-phase reactions.
The first reaction is a solid-phase diffusion reaction of a Ti film with a single-crystal silicon (Si) region or a polysilicon film contacted with the Ti film.
The second reaction is the structural phase transformation from the C49 phase to the C54 phase of TiSi.sub.2. The C49 phase is a non equilibrium phase and has a relatively higher resistivity of 2.times.10.sup.-4 .OMEGA..multidot.cm, and the C54 phase is an equilibrium phase and has a relatively lower resistivity of 1.5.times.10.sup.-5 .OMEGA..multidot.cm. If such the two reactions are not performed sufficiently, a satisfactorily low sheet resistance of TiSi.sub.2 cannot be obtained.
A native oxide film of Si, i.e., a silicon dioxide (SiO.sub.3) film is one of the hindering or obstructing factors to the reactions. This SiO.sub.2 film restrains, in general, the solid-phase reaction between a Ti film formed by sputtering and a Si material contacted therewith. Accordingly, a resultant TiSi.sub.2 film not only has an unsatisfactory uniformity in thickness but also contains oxygen (O).
Additionally, the oxygen contained in the Ti silicide film causes a problem in that the oxygen restrains the diffusion of the Ti and Si atoms that occurs simultaneously with the C49/C54 phase transformation, raising the phase-transformation temperature. As the TiSi.sub.2 film becomes thinner, the oxygen contained in the TiSi.sub.2 film increases in concentration.
Hence, it is especially important for the formation process of the TiSi.sub.2 film to prevent the native SiO.sub.2 film from occurring and to restrain the oxygen from doping.
Conventionally, to avoid the effects of the native SiO.sub.2 film, a method using an etching process was developed, which is disclosed in the Japanese Non-Examined Patent Publication No. 4-226024 (August 1992). This method is termed a first conventional method here.
In this method, the native SiO.sub.2 film is removed by plasma etching in the way shown in FIGS. 1A to 1E.
First, as shown in FIG. 1A, a gate SiO.sub.2 film 12 is selectively formed on the surface of a single-crystal Si substrate 11 and then, a gate electrode 13 made of a patterned polysilicon film is formed on the film 12.
Next, a pair of lightly-doped diffusion regions are formed in the substrate 11 in self-alignment to the gate electrode 13 by ion-implantation. A pair of sidewall spacers 14 are formed on the gate SiO.sub.2 film 12 at each side of the gate electrode 13.
Subsequently, a pair of heavily-doped diffusion regions are formed in the substrate 11 in self-alignment to both of the gate electrode 13 and the sidewall spacers 14 by ion-implantation. Thus, a pair of source/drain regions 15 with the Lightly-Doped Drain (LDD) structure are formed in the substrate 11, as shown in FIG. 1A.
As a pretreatment of deposition of a Ti film, the substrate 11 is immersed into a mixture solution of sulfuric acid (H.sub.2 SO.sub.4) and hydrogen peroxide (H.sub.2 O.sub.2). Then, the uncovered surfaces of the source/drain regions 15 and the uncovered surface of the gate electrode 13 are etched by using a 10% hydrofluoric acid (HF) solution in order to clean the uncovered surfaces.
After this etching process, the substrate 11 is taken out of the HF solution. At this time, these uncovered surfaces thus cleaned of the substrate 11 are exposed to the atmosphere, so that they are oxidized by oxygen contained in the atmosphere, resulting in a native SiO.sub.2 film 16 on the uncovered surfaces, respectively, as shown in FIG. 1A.
Following this, the substrate 11 is placed on a holder in a vacuum chamber of a radio-frequency (rf) plasma etching system. In this system, the native SiO.sub.2 film 16 is entirely removed from the substrate 11 using the rf-plasma generated in the chamber, as shown in FIG. 1B. Thus, all of the uncovered surfaces of the source/drain regions 15 and the gate electrode 13 are cleaned. An applied power is about 20 to 50 W and a reaction gas is nitrogen fluoride (NF.sub.2) during this etching process.
A Ti film 17 is then formed on the entire substrate 11 by sputtering, as shown in FIG. 1C. The Ti film 17 is contacted with the uncovered surface of the gate electrode 13 and those of the source/drain regions 15, respectively. This film formation process of Ti is carried out under a vacuum to keep the uncovered surfaces clean.
The substrate 11 with the Ti film 17 is subjected to a first heat-treatment process either in an inert gas atmosphere of nitrogen (N) or argon (Ar), or in vacuum. Through this process, the element Ti in the Ti film 17 reacts with the element Si in the polysilicon gate electrode 13 and in the single-crystal Si source/drain regions 15 at their contact areas. As a result, TiSi.sub.2 films 18 with the C49 structures or phases are formed at an interface of the Ti film 17 and the gate electrode 13 and at interfaces of the film 17 and the source/drain regions 15, respectively.
After the unreacted Ti film 17 is removed from the substrate 11 by etching, the substrate 11 is subjected to a second heat-treatment process at 800.degree. to 900.degree. C. in the same atmosphere as that of the first heat-treatment. Through this process, the TiSi.sub.2 films 18 with the C49 structures are transformed in phase to TiSi.sub.2 films 19 with C54 structures or phases. Since each C54-phase TiSi.sub.2 film 19 has a relatively lower resistivity than that of each C49-phase TiSi.sub.2 film 18, this phase transformation realizes the sheet resistance reduction of TiSi.sub.2.
Another conventional method using a hydrogen gas (H.sub.2) for removing the native SiO.sub.2 film 16 was developed and described in the Japanese Non-Examined Patent Publication No. 3-263830 (November 1991). This method was developed to be employed to the epitaxial growth technology of semiconductor using a molecular-beam epitaxy (MBE) system. Also, this method is termed a second conventional method here.
In this second conventional method, an H.sub.2 gas is heated up to 1000.degree. C. or higher in a first vacuum chamber with a pressure of 1.times.10.sup.-3 Torr to generate atomized hydrogen species. Also, a single-crystal Si substrate is placed on a holder in a second vacuum chamber whose pressure is lower than that of the first chamber, i.e., 1.times.10.sup.-4 Torr. The atomized hydrogen species in the first chamber are then supplied to the second chamber to be irradiated to the substrate, removing a native SiO.sub.2 film produced on the uncovered surface of the substrate.
The chemical reaction of the native oxide removal is EQU H.sub.2 +SiO.sub.2 -H.sub.2 O+SiO.
The native SiO.sub.2 film is reduced or deoxidized by H.sub.2 to be evaporated from the substrate in the form of SiO. During this process, the partial pressure of SiO needs to be kept higher than the total pressure of the second vacuum chamber.
With the first conventional method using the plasma etching as shown in FIGS. 1A to 1E, there is a disadvantage that energetic ionic species such as N atoms contained in the plasma are doped into the substrate during the native oxide film removing process.
Similarly, with the second conventional method using the hydrogen reduction, the atomized hydrogen species are accelerated to be irradiated to the substrate utilizing the pressure difference between the first and second vacuum cambers. Accordingly, there is a disadvantage that the hydrogen atoms are doped into the substrate during the native oxide removing process.
The impurity atoms doped into the substrate such as N and H atoms raises the phase transformation temperature because the impurity atoms restrain the Ti and Si atoms from diffusing inside the TiSi.sub.2 film. This is stated in detail in the Journal of Applied Physics, Vol. 70, No. 5, pp 2660-2666, September 1991.
From this paper, the above problem of increase of the phase transformation temperature can be solved by high-temperature annealing at a higher temperature than the phase transformation temperature. However, in the case of such the high-temperature annealing, there arises another problem that the TiSi.sub.2 film tends to be discontinuous due to its agglomeration.
The reason of the discontinuity is considered as follows:
When the TiSi.sub.2 film is heated up to 800.degree. C. or higher, to reduce its resistivity through a phase transformation it starts to soften, being capable of flowing. The softened because of the impurity atoms doped into the TiSi.sub.2 film; film tends to flow toward the surface of the TiSi.sub.2 film and/or the interface of the TiSi.sub.2 film and the Si material, so that the structure or constitution of the TiSi.sub.2 film varies so as to minimize its internal energy. As a result, the TiSi.sub.2 film becomes partially discontinuous, generating continuous regions and isolated island regions. This means that the TiSi.sub.2 film tends to have both breakdown or snapping and loss of the thickness uniformity, so that the film deteriorates in conductivity and increases in sheet resistance.
Accordingly, it is seen that the above high temperature annealing is not suitable for the above resistance reduction purpose.
The above discontinuity of the TiSi.sub.2 film due to agglomeration is stated in detail in the Journal of Applied Physics, Vol. 71, No. 2, pp 720-724, January 1992.
As described above, in view of the above resistance reduction purpose, the C54-phase TiSi.sub.2 film with a relatively lower resistivity can be obtained only in the temperature range between the phase transformation temperature and the agglomeration temperature.
Therefore, a fabrication method that can produce such the low-resistivity TiSi.sub.2 film without the above disadvantages has been required in order to realize finely-structured semiconductor devices.